Get a new one.
There are two things you can do:
Take the car to a lock smith who will be able to take the drum apart and cut you a new key to match it, its the most expensive, but also the easiest.
or,
Find another wrecked spitfire, or Herald, or other Triumph or possibly an Austin from the 60s or early 70s and take the ignition barrel and door locks from it with the matching key.
The problem is depending on where you are in the world finding a suitable donor, and then hoping it still has the orginal matching keys with it.
And of course you then have to spend the time fitting the door and ignition locks.
lots of fun, but if you do it yourself the cheapest option.
